The present invention relates to a method of promoting the transport of water through medium and coarse grained soils.
Soil particles contain a large number of small channels or capillaries through which water is capable of flowing, and may be graded on the basis of the capillary or pore diameters. As water is made to flow through a channel, whether that channel be a soil pore or not, the rate of capillary water flow through the channel will be higher if the water is capable of wetting the channel surface. At the interface of the water and the capillary surface, however, there exists a long range van der Waal interaction between the water and the capillary surface. While the van der Waals interaction typically extends less than 200 angstroms into the body of water, it nonetheless decreases the ability of the water to wet the capillary surface, thereby increasing the contact angle between the water and the capillary surface and hindering the flow of water therethrough. While the negative effect of the van der Waals interaction may be negligible in the case of water flowing through a household pipe, when one considers the flow of water through minute soil pores, this interaction has a major effect.
It is well known that certain water-soluble polymers, when placed within a soil environment, dramatically alter the flow of moisture through the soil and increase water retention. Among the water-soluble polymers utilizable for this purpose are high molecular weight poly(ethylene oxide), polyvinyl alcohol, polyvinyl pyrrolidone and polyacrylamide (whether hydrolyzed or not). The use of these polymers is suggested in such patents as U.S. Pat. Nos. 3,633,310; 3,798,838; 3,909,228; and Japanese Pat. No. 47-2528 (1972). U.S. Pat. No. 4,163,657 describes a soil conditioning composition (preferably a substituted poly(ethylene oxide)) having enhanced retention time within the soil. It will be appreciated that these linear, water-soluble, hydrophilic polymers directly control the physical properties of the soil water by modifying, e.g., its viscosity, surface tension, and contact angle, and hence act in an entirely different manner than the cross-linked, water-insoluble hydrophilic "super-slurper" polymers or polymers used to aggregate soil particles. The aforementioned Japanese patent teaches that the poly(ethylene oxide) should have a molecular weight of 300,000 to 5,000,000 and should be applied at 50-500 parts per million by weight of dry soil. The aforementioned U.S. Pat. No. 4,163,657 teaches that the polymer must have a molecular weight greater than 50,000 and is to be used at a level of 5-2500 parts per million parts by weight of dry soil. The patent further provides that, within the ranges specified, the amount of polymer to be used would be dependent upon a marginal cost analysis involving the price of the polymer and the increment in the value of the crop produced through its use. Thus, the patent teaches that "more is better", at least until, at the high end of the range, the marginal cost of the extra quantity of polymer is not offset by the marginal increment in market value of the additional crop produced through use of the extra quantity of polymer.
Despite the suitability of the soil conditioning composition of U.S. Pat. No. 4,163,657 for its intended use in its recommended quantities, there remains a need for a composition capable of enhancing the transport of water at only a fraction of the cost.
It is an object of the present invention to provide a method of promoting the transport of water through medium and coarse grained soils by the use of economical quantities of a soil amendment.
It is a further object of the present invention to provide such a process where the soil amendment is also a composition characterized by a low washout rate from soil, thereby rendering the composition even more cost-effective.